Normally to get a .com domain name you need to find what's called a registrar. A registrar is a company who registers the domain name on your behalf with the governing body of .com domains known as ICANN. There are many registrars on the web charging a range of prices to register a .com.
If you're expecting to to register a .com for free be careful. Although you may find many free or super low priced domain registrars there is most likely a catch to their offering. All registrars pay about the same amount to register a domain name. If you're only paying $10 or less per year to register a domain name don't expect any level of decent support to be included. Companies just can't afford to have technically talented people answer phones with single digit margins.
Once you've registered your .com domain name you'll most likely want it to work in conjunction with a website. To make a domain name work with a website you need to setup Domain Name Servers or DNS for short. When people type your domain name into a browser DNS tells your computer where to go to get the web page you're looking for. Setting up your DNS with your hosting can be confusing and tedious. Many traditional hosts will issue you an IP address on DNS address. If that's the case you then need to go into your configuration with your registrar and provide that information. The process takes up to 48 hours from the time you enter the information until full propagation.
At our company WebStarts we remove all these hassles. All you have to do to get a .com web address is sign up for an account and choose the domain you want. If the domain is available for register all the DNS and IP address are setup instantly, behind the scenes, on the fly. After about 10 minutes your domain name will be working with your website and you're in business. This is just one of the very unique and powerful features we offer at WebStarts.com. And one of the many reasons over 2.5 million people trust us with their website.
If you don't know how to get a .com I highly recommend signing up for a WebStarts account at this link.